Negative electrode of power storage device and power storage device

ABSTRACT

A mixture of amorphous PAHs and at least one of a carrier ion storage metal, a Sn compound, a carrier ion storage alloy, a metal compound, Si, Sb, and SiO 2  is used as the negative electrode active material. The theoretical capacity of amorphous PAHs greatly exceeds that of a graphite-based carbon material. Thus, the use of amorphous PAHs enables the negative electrode active material to have a higher capacity than in the case of using the graphite-based carbon material. Further, addition of at least one of the carrier ion storage metal, the Sn compound, the carrier ion storage alloy, the metal compound, Si, Sb, and SiO 2  to the amorphous PAHs enables the negative electrode active material to have a higher capacity than the case of only using the amorphous PAHs.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a negative electrode of a power storagedevice and a power storage device having the negative electrode.

2. Description of the Related Art

With an increasing concern for the environmental issues, power storagedevices such as secondary batteries and electric double layer capacitorsused for power supply for hybrid vehicles and the like have beenactively developed. As the power storage devices, lithium (Li)-ionsecondary batteries and Li-ion capacitors which have high energyperformance have attracted attention. The Li-ion secondary battery,which is compact but can have a large capacity, has already been mountedon a portable information terminal such as a mobile phone or a laptoppersonal computer, thereby contributing to miniaturization of products.

The power storage device basically has a structure in which anelectrolyte is provided between a positive electrode and a negativeelectrode. It is known that each of the positive electrode and thenegative electrode includes a current collector and an active materialprovided over the current collector. For example, in a Li-ion secondarybattery, a material capable of storing and releasing Li ions is used asan active material.

Various approaches have been taken to improve the characteristics of apower storage device. For example, study of a negative electrode activematerial for a power storage device is one of the approaches to improvethe characteristics of a power storage device. A graphite-based carbonmaterial, which is mainly used as the negative electrode activematerial, has the theoretical capacity of 372 mAh/g and has already beenput to practical use with a capacity close to the theoretical capacity.Thus, an active material with a higher capacity (charge capacity) isrequired.

A material containing a semimetal, a semimetal compound, a metal, or ametal compound is given as an example of a material having a highercapacity than a graphite-based carbon material when it is used as anegative electrode active material for a power storage device. Forexample, silicon (Si) is known to have a higher capacity than agraphite-based carbon material. Patent Document 1 discloses a negativeelectrode of a Li-ion secondary battery in which a fiber shaped carbonmaterial, silicon, and a silicon compound are used in addition to agraphite-based carbon material. Patent Document 2 discloses a Li-ionsecondary battery in which a graphite-based carbon material and ametal-carbon composite material are used.

However, a negative electrode active material with a higher capacity isrequired to meet an increasing demand for a compact power storagedevice.

REFERENCE Patent Documents [Patent Document 1] Japanese Published PatentApplication No. 2004-182512 [Patent Document 2] Japanese PublishedPatent Application No. 2009-105046 SUMMARY OF THE INVENTION

An object of one embodiment of the invention is to provide a negativeelectrode active material with a higher capacity.

In order to achieve the object, in one embodiment of the invention, amixture of amorphous polycyclic aromatic hydrocarbons (PAHs) and atleast one of a carrier ion storage metal, a carrier ion storage alloy, ametal compound, Si, Sb, and SiO₂ is used as a negative electrode activematerial. Note that in this specification, “carrier ion storage metal”means a metal which can store and release carrier ions in a powerstorage device. Further, “carrier ion storage alloy” means an alloywhich can store and release carrier ions in a power storage device.

The theoretical capacity of amorphous PAHs is 1116 mAh/g and anexperimental capacity thereof is 680 mAh/g, both of which greatly exceed372 mAh/g that is the theoretical capacity of a graphite-based carbonmaterial. Therefore, in the case where amorphous PAHs are used, anegative electrode active material can have a higher capacity than inthe case where a graphite-based carbon material is used.

Further, when amorphous PAHs that are materials with a high capacity andat least one of a carrier ion storage metal, a carrier ion storagealloy, a metal compound, Si, Sb, and SiO₂ are mixed, the capacity of thenegative electrode active electrode can be higher than in the case whereonly the amorphous PAHs are used.

One embodiment of the invention is a negative electrode of a powerstorage device comprising a negative electrode active materialcontaining amorphous PAHs and at least one of a carrier ion storagemetal, a carrier ion storage alloy, a metal compound, Si, Sb, and SiO₂;and a current collector.

The carrier ion storage metal may be any one of Sn, Al, Zn, and Bi. Thecarrier ion storage alloy may be any one of alloys expressed by a Sn-Malloy (M is Fe, Co, Mn, V, or Ti). Further, as the metal compound, a Sncompound or a metal compound used as a positive electrode material in astate where carrier ions are released (a decarrierionized state) can beused. The Sn compound may be any one of SnO₂, Sn₂P₂O₇, and SnPBO₆.Further, the metal compound used as the positive electrode material in astate where carrier ions are released (a decarrierionized state) may beany one of SnPO₄ClCoO, NiO, MnO₂, and FePO₄.

In the above, the carrier ion may be either of a Li ion and a Na ion.

Further, the amorphous PAHs may have a spherical shape.

Furthermore, at least one of a metal, a metal compound, Si, Sb, and SiO₂may be attached to the surfaces of the amorphous PAHs.

Furthermore, the amorphous PAHs may contain at least one of a metal, ametal compound, Si, Sb, and SiO₂ at greater than or equal to at 1 wt %and less than or equal to 50 wt %.

Furthermore, one embodiment of the invention is a power storage deviceincluding the negative electrode, a positive electrode, and anelectrolyte solution containing an electrolyte.

According to one embodiment of the invention, a negative electrodeactive material with a higher capacity can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are schematic views of examples of a negative electrodeactive material and a negative electrode.

FIGS. 2A and 2B are a plan view and a cross-sectional view illustratingone embodiment of a power storage device.

FIG. 3 is a diagram illustrating application modes of a power storagedevice.

FIGS. 4A and 4B are scanning electron micrograph images eachillustrating an example of a negative electrode active material.

FIGS. 5A and 5B are scanning electron micrograph images eachillustrating an example of a negative electrode active material.

FIG. 6 shows evaluation results of an example of a negative electrode.

FIG. 7 shows evaluation results of an example of a negative electrode.

FIG. 8 shows evaluation results of an example of a negative electrode.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, Embodiments are described in detail using the drawings.Note that the invention is not limited to the following description ofthe embodiments, and it is readily appreciated by those skilled in theart that modes and details of the invention can be modified in a varietyof ways without departing from the spirit of the invention disclosed inthis specification and the like. A structure of the different embodimentcan be implemented by combination appropriately. On the description ofthe invention with reference to the drawings, a reference numeralindicating the same part is used in common throughout differentdrawings, and the repeated description is omitted.

Note that the position, the size, the range, or the like of eachstructure illustrated in the drawings and the like is not accuratelyrepresented in some cases for easy understanding. Therefore, the presentinvention is not necessarily limited to the position, size, range, orthe like disclosed in the drawings and the like.

Embodiment 1

In this embodiment, a negative electrode active material for a powerstorage device, which is one embodiment of the invention, and amanufacturing method thereof will be described with reference to FIGS.1A to 1C.

Negative Electrode Active Material

An example of a negative electrode active material 100 will be describedwith reference FIG. 1A. The negative electrode active material 100includes amorphous PAHs 101 and a fine particle 102 formed of any one ofa metal, a metal compound, Si, Sb, and SiO₂. In addition, the negativeelectrode active material 100 may include a secondary particle 103 inwhich a plurality of the fine particles 102 is aggregated.

As the amorphous PAHs 101, PAHs with a hydrogen/carbon atomic ratio(hereinafter referred to as a H/C ratio) of 0.05 or more and 0.5 or lessare used. As the amorphous PAHs with a H/C ratio of 0.05 or more and 0.5or less, for example, a polyacenic material or a hard carbon-basedmaterial can be used.

The polyacenic material has a higher capacity (about 850 mAh/g) than thegraphite-based carbon material. Further, the hard carbon-based materialhas a higher capacity (about 400 mAh/g to 700 mAh/g) than thegraphite-based carbon material and has discharge characteristics suchthat voltage uniformly descends to a discharge end voltage. Thus, any ofthese materials is preferably used for the negative electrode activematerial because in that case a power storage device has a high energydensity.

When the amorphous PAHs 101 have a spherical shape, variation in thecontact area with other constituent elements of the negative electrodecan be reduced. Thus, the amorphous PAHs preferably have a sphericalshape because variation in resistance in the negative electrode activelayer can be reduced; further, because there is little abrasion when itis transferred, high-density filling is easily achieved, and fluidity inthe case of being mixed with other constituent elements of the negativeelectrode can be improved. Specifically, a grain diameter of theamorphous PAHs 101 is preferably 100 μm or less.

Note that in this specification and the like, “spherical shape” does notnecessarily mean an accurate spherical shape. For example, asubstantially spherical shape (for example, the smallest diameter isgreater than or equal to 70% and less than 100% of the longestdiameter), a deformed spherical shape, a spherical shape with aprojection on its surface, and an elliptical spherical shape areincluded.

In this embodiment, for the amorphous PAHs 101, a spherical polyacenicmaterial is used.

In this embodiment, a metal, a metal compound, Si, Sb, or SiO₂ is mixedto the amorphous PAHs 101.

A carrier ion storage metal is used as the metal to be mixed.Alternatively, a carrier ion storage alloy or a metal compound is usedas the metal compound to be mixed.

As carrier ions, alkali metal ions such as Li ions or sodium (Na) ions,alkaline earth metal ions, beryllium (Be) ions, magnesium (Mg) ions, orthe like can be used. The use of Li ions as the carrier ions enables apower storage device to have a small memory effect, a high energydensity, a high charge/discharge capacity, and a high output voltage,which is preferable. Further, Na ions that are abundant in resources arepreferably used because the cost of manufacturing a power storage devicecan be reduced. In this embodiment, Li ions are used as the carrierions.

As the carrier ion storage metal to be mixed, a metal having propertiesof storing carrier ions, for example, Sn, Al, Zn, or Bi can be used.

As the carrier ion storage alloy to be mixed, an alloy having propertiesof storing carrier ions, for example, an alloy represented by Sn-M (M isFe, Co, Mn, V, or Ti) can be used.

As the metal compound to be mixed, a Sn compound or a metal compoundused as a positive electrode material in a state where carrier ions arereleased (a decarrierionized state) can be used. As the Sn compound, forexample, SnO₂, Sn₂P₂O₇, SnPBO₆, or SnPO₄Cl can be used. Further, as themetal compound used as a positive electrode material in a state wherecarrier ions are released (a decarrierionized state), a stable substanceeven in the absence of carrier ions, among substances that can be usedas a positive electrode active material, for example, CoO, NiO, MnO₂,NiMnO₄, or FePO₄ can be used.

The fine particle 102 is a particle composed of a metal, a metalcompound, Si, Sb, or SiO₂. The fine particle 102 preferably has a graindiameter of for example, 1 μm or less in order to increase theefficiency of reaction with the carrier ion.

The fine particle 102 composed of a metal, a metal compound, Si, Sb, orSiO₂ may be, but is not necessarily, attached to the outer surface ofthe amorphous PAHs 101. Further, the secondary particle 103 may begenerated from the particle 102 composed of a metal, a metal compound,Si, Sb, or SiO₂. The secondary particle 103 may be, but is notnecessarily, attached to the outer surface of the amorphous PAHs 101.Note that it is more preferable that the fine particle 102 composed of ametal, a metal compound, Si, Sb, or SiO₂ be attached to the PAHs 101without being aggregated. This is because the fine particle 102 has alarger specific surface area than the aggregated secondary particles 103and easily reacts with carrier ions. When the fine particle 102 isattached to the amorphous PAHs 101, the secondary particle is not easilyformed.

It is preferable that the amorphous PAHs 101 contain any one of a metal,a metal compound, Si, Sb, and SiO₂ at greater than or equal to 1 wt %and less than or equal to 50 wt %, preferably greater than or equal to 1wt % and less than or equal to 30 wt %. It is not preferable that theamount of a metal, a metal compound, Si, Sb, or SiO₂ be too small,because the effect of making the power storage device having a highcapacity is decreased. Also, it is not preferable that the amount of ametal, a metal compound, Si, Sb, or SiO₂ be too large, because electricconductivity of the negative electrode active material is too low. Inthis embodiment, the amorphous PAHs 101 contain SiO₂ at greater than orequal to 1 wt % and less than or equal to 30 wt %.

Negative Electrode

An example of a negative electrode 200 of a power storage device that isone embodiment of the present invention will be described with referenceto FIGS. 1B and 1C.

The negative electrode 200 in FIG. 1B includes the negative electrodeactive material 100, a conduction auxiliary agent 120, and a binder (notshown in FIG. 1B) over a negative electrode current collector 130.

A conductive material such as copper (Cu), titanium (Ti), aluminum (Al),or stainless steel, which is processed into a foil shape, a plate shape,a net shape, or the like can be used for the negative electrode currentcollector 130.

An electron-conductive material which does not cause chemical change inthe power storage device is used for the conduction auxiliary agent 120.For example, graphite; a carbon particle; a carbon fiber; a metalmaterial such as Cu, nickel (Ni), Al, or silver (Ag); or powder, fiver,and the like of mixtures thereof can be used. In the negative electrode200 in FIG. 1B, acetylene black that is one of carbon particles is used.

The binder exists between the negative electrode active material 100,the conduction auxiliary agent 120, and the current collector 130, andthese substances are bonded one another by the binder. As the binder,polysaccharides such as starch, carboxymethyl cellulose, hydroxypropylcellulose, regenerated cellulose, and diacetyl cellulose; vinyl polymerssuch as polyvinyl chloride, polyethylene, polypropylene, polyvinylalcohol, polyvinyl pyrrolidone, polytetrafluoroethylene, polyvinylidenefluoride, ethylene-propylene-diene monomer (EPDM) rubber, sulfonatedEPDM rubber, styrene-butadiene rubber, butadiene rubber, and fluorinerubber; polyether such as polyethylene oxide; and the like can be given.In this embodiment, PVdF is used.

Note that the negative electrode active material layer may be predopedwith carrier ions. When Li ions are used as the carrier ions, a Li layeris formed on a surface of the negative electrode active material layerby a sputtering method. Alternatively, Li foil is provided on thesurface of the negative electrode active material layer, whereby thenegative electrode active material layer can be predoped with Li.

Further, as shown in FIG. 1C, graphene or multilayer graphene 121 may beused instead of the conduction auxiliary agent 120 and the binder. Theuse of graphene or multilayer graphene 121 can suppress adverse effects(pulverization of the negative electrode active material 100 andseparation of the negative electrode active material layer) of expansionand contraction of the negative electrode active material 100 due tostoring and releasing of carrier ions. Further, graphene or themultilayer graphene 121 stores carrier ions and functions as a negativeelectrode active material and thus a negative electrode with a highercapacity can be obtained.

Method for Manufacturing Negative Electrode Active Material

An example of a method for manufacturing a negative electrode activematerial will be described below.

First, materials of amorphous PAHs are prepared. When a polyacenicmaterial is used as the amorphous PAHs 101, for example, a phenol resincan be used for raw materials of the polyacenic material. When thepolyacenic material is used, a baking temperature at the time of bakingthat is to be described later can be low; thus, productivity isimproved.

When a hard carbon-based material is used as the amorphous PAHs 101, forexample, a furfuryl alcohol resin, a saccharide such as saccharose orcellulose can be used for raw materials of the hard carbon-basedmaterial.

Note that cleaning is preferably performed to remove an organic impurityattached to the amorphous PAHs 101. For example, ultrasonic cleaning inan organic solvent may be conducted.

Next, a metal, a metal compound, Si, Sb, or SiO₂ is mixed to theamorphous PAHs 101. As for a metal, a metal compound, Si, Sb, or SiO₂,the description of FIG. 1A can be referred to.

In this embodiment, a polyacenic material is used as the amorphous PAHs101, and a spherical phenol resin is used for the raw materials of thepolyacenic material. A fine particle of SiO₂ is mixed to the sphericalphenol resin.

There is no limitation on a method for mixing, and for example, adry-mixing method can be performed. In the case where a spherical phenolresin is used, a method in which the shape of the resin can bemaintained is preferably employed; for example, mixing with the use of arotatable roller is preferably employed.

Next, a mixture of the material of the amorphous PAHs 101 and at leastone of a metal, a metal compound, Si, Sb, and SiO₂ is baked. The bakingis preferably performed under an inert atmosphere, for example, anitrogen atmosphere. The temperature and time at the baking may be setunder sufficient conditions for carbonization of the amorphous PAHs 101.In the case of using a phenol resin, for example, the baking temperaturecan be set to greater than or equal to 600° C. and less than or equal to800° C. There is no limitation on the method of baking; for example,baking using a muffle furnace can be performed.

By the above method, the negative electrode active material 100 that isone embodiment of the invention can be manufactured.

Method for Manufacturing Negative Electrode

An example of a method for manufacturing the negative electrode 200including the negative electrode active material 100 will be describedbelow.

First, the negative electrode active material 100, the conductionauxiliary agent 120, and the binder are mixed using a solvent, so thatslurry is formed. There is no particular limitation on the solvent, forexample, an organic solvent such as N-methyl-2-pyrrolidone (NMP) can beused.

Note that graphene or the multilayer graphene 121 may be used instead ofthe conduction auxiliary agent 120 and the binder. Note that in thisspecification, graphene refers to a one-atom-thick sheet of carbonmolecules having sp² bonds. Further, multilayer graphene refers to astack of 2 to 100 sheets of graphene, and may contain less than or equalto 30 at. % of an element other than carbon, such as oxygen or hydrogen.Alternatively, the multilayer graphene may contain less than or equal to15 at. % of an element other than carbon and hydrogen. Note that analkali metal such as Li, Na, or potassium (K) may be added to grapheneor multilayer graphene.

Next, the slurry is applied onto the negative electrode currentcollector 130. An anchor coat may be applied before the slurry isapplied to the negative electrode current collector 130 so as to improveadhesion between the negative electrode current collector 130 and thenegative electrode active material 100. Further, the slurry containingthe negative electrode active material 100 may be applied to one surfaceof the negative electrode current collector 130 as shown in FIGS. 1B and1C or both surfaces thereof.

Next, after the negative electrode current collector 130 and the slurryare dried to form the negative electrode 200 into a desired shape, thenegative electrode 200 is further dried.

Through the above steps, the negative electrode 200 that is oneembodiment of the invention can be manufactured.

Embodiment 2

In this embodiment, an example of a power storage device that is oneembodiment of the present invention will be described with reference toFIGS. 2A and 2B.

The power storage device that is one embodiment of the present inventionincludes at least a positive electrode, a negative electrode, aseparator, and an electrolyte solution. The negative electrode is theone described in Embodiment 1.

The electrolyte is a nonaqueous solution containing an electrolyte saltor a solution containing an electrolyte salt. Any electrolyte salt canbe used as the electrolyte salt as long as it contains carrier ions suchas alkali metal ions, alkaline earth metal ions, Be ions, or Mg ions.Examples of the alkali metal ions include Li ions, Na ions, and K ions.Examples of the alkaline earth metal ions include calcium (Ca) ions,strontium (Sr) ions, and barium (Ba) ions. In this embodiment, theelectrolyte salt is an electrolyte salt containing Li ions (hereinafter,referred to as a Li-containing electrolyte salt).

With the above structure, the power storage device can be a secondarybattery or a capacitor. Further, an electric double layer capacitor canbe obtained by using only a solvent for an electrolyte solution withoutusing the electrolyte salt.

Here, the power storage device will be described with reference to thedrawing.

FIG. 2A shows a structural example of a power storage device 351. FIG.2B is a cross-sectional view along dashed dotted line X-Y in FIG. 2A.

The power storage device 351 shown in FIG. 2A includes a power storagecell 355 in an exterior member 353. The power storage device 351 furtherincludes terminal portions 357 and 359 which are connected to the powerstorage cell 355. For the exterior member 353, a laminate film, apolymer film, a metal film, a metal case, a plastic case, or the likecan be used.

As shown in FIG. 2B, the power storage cell 355 includes a negativeelectrode 363, a positive electrode 365, a separator 367 between thenegative electrode 363 and the positive electrode 365, and anelectrolyte 369 with which the exterior member 353 is filled.

The negative electrode 363 is the one described in Embodiment 1. Thenegative electrode current collector 371 is connected to the terminalportion 359. A positive electrode current collector 375 is connected tothe terminal portion 357.

Further, the terminal portions 357 and 359 each partly extend outsidethe exterior member 353.

The positive electrode layer 365 is formed to include a positiveelectrode current collector 375 and a positive electrode active materiallayer 377. The positive electrode active material layer 377 is formed onone or both surfaces of the positive electrode current collector 375.Further, the positive electrode 365 may include a binder, a conductionauxiliary agent, and the like besides the positive electrode currentcollector 375 and the positive electrode active material layer 377.

Although a sealed thin power storage device is described as the powerstorage device 351 in this embodiment, the external shape of the powerstorage device 351 is not limited thereto. A power storage device havingany of a variety of shapes, such as a button power storage device, acylindrical power storage device, or a rectangular power storage devicecan be used as the power storage device 351. Further, although thestructure where the positive electrode, the negative electrode, and theseparator are stacked is described in this embodiment, a structure wherethe positive electrode, the negative electrode, and the separator arerolled may be employed.

For the positive electrode current collector 375, a conductive materialsuch as Al or stainless steel which is processed into a foil shape, aplate shape, a net shape, or the like can be used. Alternatively, aconductive layer provided by deposition separately on a substrate andthen separated from the substrate can be used as the positive electrodecurrent collector 375.

The positive electrode active material layer 377 can be formed usingLiFeO₂, LiCoO₂, LiNiO₂, LiMnO₄, LiFePO₄, LiCoPO₄, LiNiPO₄, LiMn₂PO₄,V₂O₅, MnO₂, or another Li compound as a material. Note that when carrierions are alkali metal ions other than Li ions, alkaline earth metalions, Be ions, or Mg ions, the positive electrode active material layer377 can be formed using, instead of Li in the above Li compounds, analkali metal (e.g., Na or K), an alkaline earth metal (e.g., Ca, Sr, orBa), Be, or Mg. For example, when carrier ions are Na ions,NaNi_(0.5)Mn_(0.5)O₂ can be used.

The positive electrode active material layer 377 is formed over thepositive electrode current collector 375 by a coating method or aphysical vapor deposition method (e.g., a sputtering method), wherebythe positive electrode 365 can be formed. In the case where a coatingmethod is employed, the positive electrode 365 is formed in such amanner that a paste in which a conduction auxiliary agent (e.g.,acetylene black), a binder (e.g., PVDF), or the like is mixed with anyof the above materials for the positive electrode active material layer377 is applied to the positive electrode current collector 375 anddried. In this case, the positive electrode 365 is preferably molded byapplying pressure as needed.

The positive electrode active material layer 377 may be formed using apaste of a mixture of the positive electrode active material andgraphene or multilayer graphene instead of a conductive auxiliary agentand a binder.

The use of graphene or multilayer graphene instead of a conductiveauxiliary agent and a binder leads to a reduction in amount of theconductive auxiliary agent and the binder in the positive electrode 365.In other words, the weight of the positive electrode 365 can be reduced;accordingly, the charge/discharge capacity of the power storage deviceper unit weight of the negative electrode can be increased.

Note that strictly speaking, “positive electrode active material” or“negative electrode active material” refers only to a material thatrelates to storing and releasing of ions functioning as carriers. Inthis specification, however, in the case of using a coating method toform an active material layer, for the sake of convenience, the activematerial layer collectively refers to the materials of the activematerial layer, that is, a substance that is actually an “activematerial”, a conductive auxiliary agent, a binder, and the like.

The electrolyte 369 is a nonaqueous solution containing an electrolytesalt or a solution containing an electrolyte salt. In particular, in aLi-ion secondary battery, a Li-containing electrolyte salt in which Liions as carrier ions can transfer and stably exist is used. Examples ofthe Li-containing electrolyte salt includes LiClO₄, LiAsF₆, LiBF₄,LiPF₆, and Li(C₂F₅SO₂)₂N. Note that when carrier ions are alkali metalions other than Li ions or alkaline earth metal ions, alkali metal salt(e.g., Na salt or K salt), alkaline earth metal salt (e.g., Ca salt, Srsalt or Ba salt), Be salt, Mg salt, or the like can be used for thesolute of the electrolyte 369. For example, when Na ions are used as thecarrier ions, NaPF₆, NaClO₄, or the like can be used as the solute(electrolyte salt).

The electrolyte 369 is preferably a nonaqueous solution containing anelectrolyte salt. That is, as a solvent of the electrolyte 369, anaprotic organic solvent is preferably used. Examples of the aproticorganic solvent include ethylene carbonate, propylene carbonate,dimethyl carbonate, diethyl carbonate, γ-butyrolactone, acetonitrile,dimethoxyethane, and tetrahydrofuran, and one or more of these materialscan be used. Alternatively, as the aprotic organic solvent, one ionicliquid or a plurality of ionic liquids may be used. Owing tonon-flammability and non-volatility of an ionic liquid, it is possibleto suppress explosion, inflammation, and the like of the power storagedevice 351 at the time when the internal temperature of the powerstorage device 351 rises, resulting in improvement in safety.

When a gelled high-molecular material containing an electrolyte salt isused as the electrolyte 369, safety against liquid leakage and the likeis improved and the power storage device 351 can be thinner and morelightweight. Examples of the gelled high-molecular material include asilicon gel, an acrylic gel, an acrylonitrile gel, polyethylene oxide,polypropylene oxide, and a fluorine-based polymer.

As the electrolyte 369, a solid electrolyte such as Li₃PO₄ can be used.

As the separator 367, an insulating porous material is used. Forexample, paper; nonwoven fabric; a glass fiber; a synthetic fibercontaining nylon (polyamide), vinylon (polyvinyl alcohol based fiber),polyester, acrylic, polyolefin, or polyurethane; or ceramics may beused. Note that a material which does not dissolve in the electrolyte369 should be selected.

In the case where the power storage device that is one embodiment of thepresent invention is a Li-ion capacitor, instead of the positiveelectrode active material layer 377, a material capable of reversiblystoring and releasing one of or both Li ions and anions may be used.Examples of the material include active carbon, graphite, a conductivepolymer, and a polyacenic material.

With the use of the negative electrode active material, a power storagedevice with a high capacity can be obtained.

Note that this embodiment can be implemented in appropriate combinationwith any of the structures of the other embodiments and example.

Embodiment 3

The power storage device that is one embodiment of the present inventioncan be used for power supplies of a variety of electric and electronicdevices which are operated with power.

Specific examples of electric and electronic devices each utilizing thepower storage device that is one embodiment of the present invention areas follows: display devices, lighting devices, desktop personalcomputers and laptop personal computers, image reproduction deviceswhich reproduce still images and moving images stored in recording mediasuch as digital versatile discs (DVDs), mobile phones, portable gamemachines, portable information terminals, tablet terminals, e-bookreaders, video cameras, digital still cameras, high-frequency heatingappliances such as microwave ovens, electric rice cookers, electricwashing machines, air-conditioning systems such as air conditioners,electric refrigerators, electric freezers, electricrefrigerator-freezers, freezers for preserving DNA, and medicalelectrical and electronic equipment such as dialyzers. In addition,moving objects driven by an electric motor using electric power from apower storage device are also included in the category of electric andelectronic devices. As examples of the moving objects, electricvehicles, hybrid vehicles which include both an internal-combustionengine and a motor, motorized bicycles including motor-assistedbicycles, and the like can be given.

In the electric and electronic devices, the power storage device that isone embodiment of the present invention can be used as a power storagedevice for supplying enough electric power for almost the whole powerconsumption (such a power storage device is referred to as a main powersupply). Alternatively, in the electric and electronic devices, thepower storage device that is one embodiment of the present invention canbe used as a power storage device which can supply electric power to theelectric and electronic devices when the supply of power from the mainpower supply or a commercial power supply is stopped (such a powerstorage device is referred to as an uninterruptible power supply).Further alternatively, in the electric and electronic device, the powerstorage device that is one embodiment of the present invention can beused as a power storage device for supplying electric power to theelectric and electronic devices at the same time as the electric powersupply from the main power supply or a commercial power supply (such apower storage device is referred to as an auxiliary power supply).

FIG. 3 illustrates specific structures of the electric and electronicdevices. In FIG. 3, a display device 1000 is an example of an electronicdevice including a power storage device 1004 that is one embodiment ofthe present invention. Specifically, the display device 1000 correspondsto a display device for TV broadcast reception and includes a housing1001, a display portion 1002, speaker portions 1003, the power storagedevice 1004, and the like. The power storage device 1004 that is oneembodiment of the present invention is provided in the housing 1001. Thedisplay device 1000 can receive electric power from a commercial powersupply. Alternatively, the display device 1000 can use electric powerstored in the power storage device 1004. Thus, the display device 1000can be operated with use of the power storage device 1004 that is oneembodiment of the present invention as an uninterruptible power supplyeven when electric power cannot be supplied from the commercial powersupply due to power failure or the like.

A semiconductor display device such as a liquid crystal display device,a light-emitting device provided with a light-emitting element such asan organic EL element in each pixel, an electrophoresis display device,a digital micromirror device (DMD), a plasma display panel (PDP), afield emission display (FED), and the like can be used for the displayportion 1002.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like other than TV broadcast reception.

In FIG. 3, an installation lighting device 1100 is an example of anelectric device including a power storage device 1103 that is oneembodiment of the present invention. Specifically, the lighting device1100 includes a housing 1101, a light source 1102, a power storagedevice 1103, and the like. FIG. 3 illustrates the case where the powerstorage device 1103 is provided in a ceiling 1104 on which the housing1101 and the light source 1102 are installed; alternatively, the powerstorage device 1103 may be provided in the housing 1101. The lightingdevice 1100 can receive electric power from a commercial power supply.Alternatively, the lighting device 1100 can use electric power stored inthe power storage device 1103. Thus, the lighting device 1103 can beoperated with the use of the power storage device 1103 that is oneembodiment of the present invention as an uninterruptible power supplyeven when electric power cannot be supplied from the commercial powersupply due to power failure or the like.

Note that although the installation lighting device 1100 provided in theceiling 1104 is illustrated in FIG. 3 as an example, the power storagedevice that is one embodiment of the present invention can be used in aninstallation lighting device provided in, for example, a wall 1105, afloor 1106, a window 1107, or the like other than the ceiling 1104.Alternatively, the power storage device can be used in a tabletoplighting device and the like.

As the light source 1102, an artificial light source which emits lightartificially by using power can be used. Specifically, discharge lampssuch as an incandescent lamp and a fluorescent lamp, and alight-emitting element such as an LED and an organic EL element aregiven as examples of the artificial light source.

In FIG. 3, an air conditioner including an indoor unit 1200 and anoutdoor unit 1204 is an example of an electric device including a powerstorage device 1203 that is one embodiment of the present invention.Specifically, the indoor unit 1200 includes a housing 1201, aventilation duct 1202, the power storage device 1203, and the like. FIG.3 illustrates the case where the power storage device 1203 is providedin the indoor unit 1200; alternatively, the power storage device 1203may be provided in the outdoor unit 1204. Further alternatively, thepower storage devices 1203 may be provided in both the indoor unit 1200and the outdoor unit 1204. The air conditioner can receive power fromthe commercial power supply. Alternatively, the air conditioner can usepower stored in the power storage device 1203. Particularly in the casewhere the power storage devices 1203 are provided in both the indoorunit 1200 and the outdoor unit 1204, the air conditioner can be operatedwith the use of the power storage device 1203 that is one embodiment ofthe present invention as an uninterruptible power supply even when powercannot be supplied from a commercial power supply due to power failureor the like.

Note that although the separated air conditioner including the indoorunit and the outdoor unit is illustrated in FIG. 3 as an example, thepower storage device that is one embodiment of the present invention canbe used in an air conditioner in which the functions of an indoor unitand an outdoor unit are integrated in one housing.

In FIG. 3, an electric refrigerator-freezer 1300 is an example of anelectric device including a power storage device 1304 that is oneembodiment of the present invention. Specifically, the electricrefrigerator-freezer 1300 includes a housing 1301, a door for arefrigerator 1302, a door for a freezer 1303, and the power storagedevice 1304. The power storage device 1304 is provided in the housing1301 in FIG. 3. The electric refrigerator-freezer 1300 can receive powerfrom a commercial power supply. Alternatively, the electricrefrigerator-freezer 1300 can use power stored in the power storagedevice 1304. Thus, the electric refrigerator-freezer 1300 can beoperated with the use of the power storage device 1304 that is oneembodiment of the present invention as an uninterruptible power supplyeven when power cannot be supplied from a commercial power supply due topower failure or the like.

Note that among the electric devices described above, a high-frequencyheating apparatus such as a microwave and an electric device such as anelectric rice cooker require high electric power in a short time. Thetripping of a breaker of a commercial power supply in use of electricdevices can be prevented by using the power storage device that is oneembodiment of the present invention as an auxiliary power supply forsupplying electric power which cannot be supplied enough by a commercialpower supply.

In addition, in a time period when electric and electronic devices arenot used, particularly when the proportion of the amount of power whichis actually used to the total amount of power which can be supplied froma commercial power supply source (such a proportion referred to as ausage rate of power) is low, power can be stored in the power storagedevice, whereby the usage rate of power can be reduced in a time periodwhen the electric and electronic devices are used. For example, in thecase of the electric refrigerator-freezer 1300, power can be stored inthe power storage device 1304 in night time when the temperature is lowand the door for a refrigerator 1302 and the door for a freezer 1303 arenot often opened or closed. On the other hand, in daytime when thetemperature is high and the door for a refrigerator 1302 and the doorfor a freezer 1303 are frequently opened and closed, the power storagedevice 1304 is used as an auxiliary power supply; thus, the usage rateof power in daytime can be reduced.

In FIG. 3, a tablet terminal 1400 is an example of an example of anelectronic device including a power storage device 1403 that is oneembodiment of the present invention. Specifically, the tablet terminal1400 includes a housing 1401, a housing 1402, a power storage device1403, and the like. The housings 1401 and 1402 each have a displayportion having a touch panel function. By touching the display portionwith a finger or the like, contents displayed on the display portion canbe controlled. Further, the tablet terminal 1400 can be folded with thehousings 1401 and 1402 and the display portion placed inward; thus, thetablet terminal 1400 can be compact and the display portion can beprotected. With the use of the power storage device 1403 that is oneembodiment of the invention, the tablet terminal 1400 can be compact andused as a mobile application for a long period.

Note that this embodiment can be implemented in appropriate combinationwith any of the structures of the other embodiments and example.

Example 1

In this example, a negative electrode active material for a powerstorage device, which is one embodiment of the present invention, wasactually manufactured, and the results of evaluating the characteristicsare described with reference to FIGS. 4A and 4B, FIGS. 5A and 5B, FIG.6, FIG. 7, and FIG. 8.

Manufacture of Negative Electrode Active Material

In this example, a spherical phenol resin (“Maririn” HF-008; Gun EiChemical Industry Co., Ltd.) was used as a raw material of amorphousPAHs. An average grain diameter measured with a particle size analyzerwas 9.6 p.m.

Then, SiO₂ was mixed to the raw material of the amorphous PAHs. Notethat SiO₂ nanopowder (manufactured by Sigma-Aldrich Corporation) with agrain diameter of 10 nm to 20 nm was used as SiO₂.

First, ultrasonic cleaning was conducted in acetone so as to removeorganic impurities attached to the spherical phenol resin.

Next, SiO₂ nanopowder was added to the cleaned spherical phenol resin,and dry-mixing was performed using a rotatable roller. As shown in Table1, the additive amounts of SiO₂ nanopowder to 5 g of the sphericalphenol resin were 0 wt % (0 g, (reference example), 1 wt % (0.05 g), 10wt % (0.50 g), 20 wt % (1.00 g), and 30 wt % (1.50 g).

TABLE 1 Baking Material Yield 0 wt % addition Spherical phenol resin (5g) 51 (Ref.) 1 w % addition Spherical phenol resin (5 g) + SiO₂ (0.05 g)52 20 wt % addition Spherical phenol resin (5 g) + SiO₂ (1.00 g) 57 30wt % addition Spherical phenol resin (5 g) + SiO₂ (1.50 g) 59

Next, the mixture of the spherical phenol resin and SiO₂ nanopowder werebaked so as to be a negative electrode active material. The baking wasperformed at 700° C. using a muffle furnace under a nitrogen atmosphere(N₂, 5 L/min) for 10 hours. Table 1 shows the weight yield after thebaking.

FIGS. 4A and 4B and FIGS. 5A and 5B each show scanning electronmicrographs of the negative electrode active materials manufactured inthe above manner. FIG. 4A shows the negative electrode active materialin which 0 wt % of SiO₂ nanopowder (reference example) was added to thespherical phenol resin; FIG. 4B shows the negative electrode activematerial in which 1 wt % of SiO₂ nanopowder was added to the sphericalphenol resin; FIG. 5A shows the negative electrode active material inwhich 20 wt % of SiO₂ nanopowder was added to the spherical phenolresin; and FIG. 5B shows the negative electrode active material in which30 wt % of SiO₂ nanopowder was added to the spherical phenol resin. Thestate where SiO₂ nanopowder was attached to the outer surface of thespherical phenol resin was observed in each of FIG. 4B, and FIGS. 5A and5B.

Manufacture of Negative Electrode

Negative electrodes were manufactured using the above-described negativeelectrode active materials. For the material of the negative electrode,in addition to the negative electrode active material, acetylene blackwas used as a conduction auxiliary agent, PVdF was used as a binder, andCu foil was used as a current collector. The combination ratio ofnegative electrode active material to acetylene black and PVdF was setto 82:8:10 (wt %).

First, the negative electrode active material and PVdF were mixed usinga homogenizer with NMP used as a solvent; then acetylene black was addedthereto to be mixed, whereby slurry was formed by adjusting theviscosity using the NMP. After an anchor coat is applied to the Cu foilcurrent collector to a thickness of about 1 μm to 2 μm, the slurry wasapplied to the Cu foil current collector.

Next, the Cu foil current collector and the slurry were dried at 70° C.for 15 minutes using a circulation drier. This was punched into a roundhole with a diameter of 16.15 mm and baked at 170° C. for 10 hours usinga vacuum furnace, whereby the negative electrode was obtained.

Table 2 shows the thicknesses and the densities of thus manufacturednegative electrodes. The negative electrodes each having a similarthickness and density were manufactured under conditions as shown inTable 2.

TABLE 2 Electrode Thickness Density  0 wt % addition (Ref.) 44.6 m 0.88g/cm³  1 wt % addition 41.8 m 0.92 g/cm³ 10 wt % addition 41.4 m 0.90g/cm³ 20 wt % addition 43.1 μm 0.93 g/cm³ 30 wt % addition 43.8 m 0.94g/cm³

Evaluation of Negative Electrode

The charge/discharge capacity and efficiency of thus manufacturednegative electrodes were measured and charge/discharge characteristicswere evaluated.

In order to evaluate the charge/discharge characteristics, a cell wasformed using thus manufactured negative electrode as a working electrodeand using Li metal with a diameter of 15 mm as the opposite electrode. Aglass fiber filter was used as a separator, and an electrolyte in which1 mol/L lithium hexafluorophosphate (LiPF₆) was dissolved in a mixedsolution of ethylene carbonate (EC) and ethylmethyl carbonate (EMC) (thevolume ratio was 3:7) was used.

A constant-current constant-voltage (CCCV) charging with a current valueof 0.2 C (1.3 mA), the lower limit voltage of 1 mV, and an end currentof 10 μA was employed for the charging. A constant current (CC)discharging with a current value of 0.2 C (1.3 mA) and the upper limitvoltage of 2 V was employed for the discharging.

FIG. 6, FIG. 7, FIG. 8, and Table 3 each show the results of evaluatingthe charge/discharge characteristics. FIG. 6 shows the charge/dischargecharacteristics of the negative electrode using the negative electrodeactive material in which only the spherical phenol resin was used (0 wt% of SiO₂ addition, reference example); FIG. 7 shows thecharge/discharge characteristics of the negative electrode using thenegative electrode active material in which 1 wt % of SiO₂ nanopowderwas added to the spherical phenol resin; and FIG. 8 shows thecharge/discharge characteristics of the negative electrode using thenegative electrode active material in which 30 wt % of SiO₂ nanopowderwas added to the spherical phenol resin. The measurement was performedusing two samples in each case. In each of the graphs, the vertical axisrepresents voltage and the horizontal axis represents capacity.

TABLE 3 Li Charge Li Discharge Capacity (mAh/g) Capacity (mAh/g)Efficiency 0 wt % addition 1011.9 412.7 40.8 (Ref.) 1076.7 433.2 40.2 1wt % addition 1116.6 460.5 41.2 1106.3 467.6 42.3 30 wt % addition 1532.9 604.5 .4 1730.0 633.2 .6

As shown in FIG. 6, in the case of using only the spherical phenol resin(0 wt % of SiO₂ addition, reference example), the maximum chargecapacity was 1076.7 mAh/g; the maximum discharge capacity was 433.2mAh/g; and the maximum efficiency was 40.8%

Further, as shown in FIG. 7, in the case of adding 1 wt % of SiO₂nanopowder to the spherical phenol resin, the maximum charge capacitywas 1116.6 mAh/g; the maximum discharge capacity was 467.6 mAh/g; andthe maximum efficiency was 42.3%.

Furthermore, in the case of adding 30 wt % of SiO₂ nanopowder to thespherical phenol resin in FIG. 8, the maximum charge capacity was 1730.0mAh/g; the maximum discharge capacity was 633.2 mAh/g; and the maximumefficiency was 39.4%.

The results revealed that the charge capacity and discharge capacitywere improved by the addition of SiO₂ nanopowder to the spherical phenolresin. Moreover, it was revealed that the charge capacity and dischargecapacity were much more improved when 30 wt % of SiO₂ nanopowder wasadded than when 1 wt % of SiO₂ nanopowder was added.

This application is based on Japanese Patent Application serial no.2011-189140 filed with Japan Patent Office on Aug. 31, 2011, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A method for manufacturing an electrode, comprising thesteps of: mixing a first particle and a second particle to obtain amixture; and baking the mixture under an inert atmosphere to form athird particle comprising a polycyclic aromatic hydrocarbon after mixingthe first particle and the second particle, wherein the second particleis attached to an outer surface of the first particle after mixing thefirst particle and the second particle, wherein the first particlecomprises one selected from the group consisting of a phenol resin, afurfuryl alcohol resin and a saccharide, and wherein the second particlecomprises one selected from the group consisting of a metal selectedfrom Si, a metal selected from Sn, Al, Zn, Sb and Bi, and a compoundcontaining Sn, Co, Ni, Mn, Fe, V or Si.
 3. The method for manufacturingthe electrode according to claim 2, further comprising the steps of:forming slurry by mixing the third particle, a binder and a solventafter baking the mixture; and applying the slurry onto a currentcollector.
 4. The method for manufacturing the electrode according toclaim 2, further comprising the step of: conducting ultrasonic cleaningof the first particle in an organic solvent before mixing the firstparticle and the second particle.
 5. The method for manufacturing theelectrode according to claim 4, wherein the organic solvent is acetone.6. The method for manufacturing the electrode according to claim 2,wherein the first particle comprises the phenol resin, and wherein thethird particle comprises a polyacenic material.
 7. The method formanufacturing the electrode according to claim 2, wherein the thirdparticle comprises a polyacenic material or a hard carbon-basedmaterial.
 8. The method for manufacturing the electrode according toclaim 2, wherein the mixing of the first particle and the secondparticle is a dry-mixing.
 9. The method for manufacturing the electrodeaccording to claim 2, wherein the second particle comprises SiO₂. 10.The method for manufacturing the electrode according to claim 2, whereinthe second particle comprises any one of SnO₂, Sn₂P₂O₇, SnPBO₆ andSnPO₄Cl.
 11. The method for manufacturing the electrode according toclaim 2, wherein the second particle comprises a compound represented bySn₂M, where M is Fe, Co, Mn, V or Ti.
 12. The method for manufacturingthe electrode according to claim 2, wherein the second particlecomprises any one of CoO, NiO, MnO₂ and FePO₄.
 13. The method formanufacturing the electrode according to claim 2, wherein the electrodeis a negative electrode for a power storage device.
 14. The method formanufacturing the electrode according to claim 2, wherein a graindiameter of the second particle is 10 nm to 20 nm.
 15. A method formanufacturing an electrode, comprising the steps of: conductingultrasonic cleaning of a first particle in an organic solvent; mixingthe first particle and a second particle to obtain a mixture; and bakingthe mixture under an inert atmosphere to form a third particlecomprising a polycyclic aromatic hydrocarbon after mixing the firstparticle and the second particle, wherein the second particle isattached to an outer surface of the first particle after mixing thefirst particle and the second particle, wherein the first particle is aspherical phenol resin particle, wherein the second particle comprisesone selected from the group consisting of a metal selected from Si, ametal selected from Sn, Al, Zn, Sb and Bi, and a compound containing Sn,Co, Ni, Mn, Fe, V or Si, and wherein an amount of the second particle tothe first particle is greater than or equal to 1 wt % and less than orequal to 50 wt %.
 16. The method for manufacturing the electrodeaccording to claim 15, further comprising the steps of: forming slurryby mixing the third particle, a binder and a solvent after baking themixture; and applying the slurry onto a current collector.
 17. Themethod for manufacturing the electrode according to claim 15, whereinthe second particle is SiO₂ powder.
 18. The method for manufacturing theelectrode according to claim 15, wherein the second particle comprisesany one of SnO₂, Sn₂P₂O₇, SnPBO₆ and SnPO₄Cl.
 19. The method formanufacturing the electrode according to claim 15, wherein the secondparticle comprises a compound represented by Sn₂M, where M is Fe, Co,Mn, V or Ti.
 20. The method for manufacturing the electrode according toclaim 15, wherein the second particle comprises any one of CoO, NiO,MnO₂ and FePO₄.
 21. The method for manufacturing the electrode accordingto claim 15, wherein the electrode is a negative electrode for a powerstorage device.